Evaluation of a Direct-Fed Microbial Product Effect on the Prevalence and Load of Escherichia coli O157:H7 in Feedlot Cattle3

Direct-fed microbials (DFM) have been identified as potential preharvest interventions for the reduction of foodborne bacterial pathogens such as Escherichia coli O157:H7. This study evaluated the efficacy of a DFM consisting of Bacillus subtilis strain 166 as an antimicrobial intervention strategy for the reduction of prevalence and load of E. coli O157:H7 in feces and on hides of feedlot cattle. Cattle (n ~ 526) were divided among 16 feedlot pens. Half of the pens received the DFM, and the other half did not. Hide and fecal samples were collected from each animal on days 28, 63, and 84 of the feeding trial. Over the course of the 84-day feeding period, there were no significant differences observed between treatments for either hide or fecal prevalence of E. coli O157:H7, or for the percentage of animals that were shedding E. coli O157:H7 at high levels ($200 CFU/g) in their feces or harboring E. coli O157:H7 at high levels ($40 CFU/cm) on their hides. In addition, there was no significant difference between the average daily gains for the treated and control groups, with both groups averaging 1.3 kg/day. We concluded that the DFM tested would not be an effective preharvest intervention against E. coli O157:H7. Much of the research to date on reducing Escherichia coli O157:H7 from the food supply has been focused on the postharvest side of the production chain. Several postharvest antimicrobial interventions (i.e., cattle or carcass rinse with various antimicrobials, steam vacuuming, steam pasteurization, etc.) have been shown to be efficacious in reducing levels of foodborne pathogens on beef carcasses and in the subsequent ground product (2, 4, 7, 11, 12, 14, 19). Preharvest interventions do not share the same wealth of techniques that have been approved for use against foodborne bacterial pathogens. A great deal of research effort is currently focused on development and validation of effective preharvest interventions (10, 22, 31). One area of research on preharvest interventions pertains to direct-fed microbials (DFM). DFM (also referred to as probiotics) have been identified as potential preharvest interventions (8, 10, 27, 34, 36). DFM are hypothesized to function by a variety of mechanisms (for example, competitive exclusion, immune modulation, or bactericidal activity via production and secretion of bacteriocins) to remove the target organism from the intestinal tract of the animal (20). For a DFM to be a useful antimicrobial intervention in the beef industry, it must act against E. coli O157:H7, a foodborne pathogen found to colonize the intestinal tracts of cattle. By reducing the E. coli O157:H7 population in the intestinal tract of feedlot cattle, the amount of E. coli O157:H7 shed in feces would be reduced, thereby potentially reducing the risk of carcass contamination by reduction of fecal-to-hide contamination at the feedlot and/or reduction of the contamination of the lairage environment at processing plants, which has been linked to carcass contamination at processing (1). This study evaluated the efficacy of a Bacillus subtilis–based DFM as an antimicrobial intervention strategy for the reduction of prevalence and load of E. coli O157:H7 in feedlot cattle. MATERIALS AND METHODS Strain. The DFM consisted of B. subtilis strain 166 made into a premix by Ivy Animal Health (Overland Park, KS). The particular strain had been isolated from corn silage and initially characterized as exhibiting broad-spectrum inhibition against gram-negative bacteria. This strain was further analyzed via in vitro experiments and found to possess small, molecule-mediated bactericidal activity against E. coli O157:H7. The premix contained B. subtilis strain 166 at 6.4 | 10 CFU/lb (,2.9 | 10/kg) with inactive carrier ingredients, 70% ground limestone (wt/wt) and 29.5% rice hulls (wt/wt). Feed. The DFM premix was added to the normal feed ration (50 lb [,22.7 kg] of premix to 7,500 lb [,3,402 kg] of feed). All animals were fed twice a day. The feeding procedure consisted of the animals in the control pens being fed the normal ration. The * Author for correspondence. Tel: 402-762-4227; Fax: 402-762-4149; E-mail: [email protected]. { Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. { Present address: IEH Laboratories and Consulting Group, 15300 Bothell Way N.E., Lake Forest Park, WA 98155, USA. 366 Journal of Food Protection, Vol. 73, No. 2, 2010, Pages 366–371